Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota.

Plants are associated with a complex microbiota that contributes to nutrient acquisition, plant growth, and plant defense. Nitrogen-fixing microbial associations are efficient and well characterized in legumes but are limited in cereals, including maize. We studied an indigenous landrace of maize grown in nitrogen-depleted soils in the Sierra Mixe region of Oaxaca, Mexico. This landrace is characterized by the extensive development of aerial roots that secrete a carbohydrate-rich mucilage. Analysis of the mucilage microbiota indicated that it was enriched in taxa for which many known species are diazotrophic, was enriched for homologs of genes encoding nitrogenase subunits, and harbored active nitrogenase activity as assessed by acetylene reduction and 15N2 incorporation assays. Field experiments in Sierra Mixe using 15N natural abundance or 15N-enrichment assessments over 5 years indicated that atmospheric nitrogen fixation contributed 29%-82% of the nitrogen nutrition of Sierra Mixe maize.

Disruption of Mediator rescues the stunted growth of a lignin-deficient Arabidopsis mutant.

Lignin is a phenylpropanoid-derived heteropolymer important for the strength and rigidity of the plant secondary cell wall. Genetic disruption of lignin biosynthesis has been proposed as a means to improve forage and bioenergy crops, but frequently results in stunted growth and developmental abnormalities, the mechanisms of which are poorly understood. Here we show that the phenotype of a lignin-deficient Arabidopsis mutant is dependent on the transcriptional co-regulatory complex, Mediator. Disruption of the Mediator complex subunits MED5a (also known as REF4) and MED5b (also known as RFR1) rescues the stunted growth, lignin deficiency and widespread changes in gene expression seen in the phenylpropanoid pathway mutant ref8, without restoring the synthesis of guaiacyl and syringyl lignin subunits. Cell walls of rescued med5a/5b ref8 plants instead contain a novel lignin consisting almost exclusively of p-hydroxyphenyl lignin subunits, and moreover exhibit substantially facilitated polysaccharide saccharification. These results demonstrate that guaiacyl and syringyl lignin subunits are largely dispensable for normal growth and development, implicate Mediator in an active transcriptional process responsible for dwarfing and inhibition of lignin biosynthesis, and suggest that the transcription machinery and signalling pathways responding to cell wall defects may be important targets to include in efforts to reduce biomass recalcitrance.

Establishment of a neutralization test involving reporter gene-expressing virus-like particles of tick-borne encephalitis virus.

Previously, a system for packaging tick-borne encephalitis virus (TBEV) subgenomic replicon RNAs into single-round infectious virus-like particles (VLPs) was developed. In the present study, VLPs were applied to measuring the levels of neutralizing antibodies against TBEV as an alternative to performing neutralization tests with live virus. As markers of VLP infection, the genes for GFP and luciferase were inserted into the TBEV replicon, which was then packaged into VLPs. The reporter genes were expressed in cells that were infected with the VLPs, and this infection was inhibited by neutralizing antibodies to TBEV. Serum samples from wild rodents were used to evaluate the neutralization test using VLPs. All the sera that were positive in the conventional neutralization test were also found to be positive in the neutralization test using VLPs, and there were highly significant correlations between the neutralization titres obtained using the native virus and those using VLPs. These results indicate that VLPs that express reporter genes represent a useful and safe alternative to conventional neutralization testing using live virus.